The Witcher 3: Wild Hunt - Complete Edition

This achievement is MISSABLE, and very easily so based on several dialogue options throughout the entire first section of the game.

Certain allies require you to do certain quests to get their support. It's important you attempt to do these before heading to the Isle of Mists.

When recruiting, you have 3 quests titled "Brothers in Arms", where you recruit your allies in the 3 major regions. Create a save here in case the achievement does not pop the first time you complete them.

Velen

Keira Metz - Probably the first one you will come across. You must complete her secondary questline. At the end of this, you will confront her about her future plans. You must tell her to go to Kaer Morhen rather than to Radovid.

Letho (Not required) - If he is alive in your save, you will find him at Reardon Manor, possibly from taking a contract or by approaching yourself. This will start his secondary quest, like Kiera, you must finish it by suggesting he goes to Kaer Morhen.


Novigrad

Zoltan - He will accept with no other requirements

Roche and Ves - Deal with Roche's problem in his secondary quest with Ves and both will accompany you.

Triss - Complete "Now or Never" and the mage's evacuation from Novigrad. The romance outcome does not matter.

Skellige

Hjalmar - Complete the "Coronation" questline, and ask either Hjalmar or Cerys to help. Either way, Hjalmar will come. (Note: If his companions Vigi and Folan survived the giant attack they will come with him, however they are not needed for the achievement)

Ermion - Like Zoltan, he will come without any further action.


These are the only ones required for the achievement. You can ask others to help in the battle such as Emhyr, Dijkstra and Crach, but they will not come and therefore do not count.